Elongated steel elements such as brass coated steel wires and steel cords are widely used to reinforce rubber products such as tires. In order to have a good adhesion formation and to reduce the rate of degradation of adhesion, particularly due to ageing in hot humid conditions, cobalt complexes are added to the rubber compound. However, cobalt is considered to be a poison for the rubber since, like most transition metals, it is an oxidation catalyst. As a result, oxidation of diene rubber molecules is accelerated, which leads to early rubber ageing. Moreover, cobalt also speeds up crack growth rate of the rubber.
In addition to the above disadvantage there is also the following problem: Cobalt is a strategic material and it is quite expensive. By adding cobalt to the whole rubber compound, one adds too much cobalt since it has only a positive function at the brass surface. Generally, it is considered that only 20% of the cobalt added to the rubber is used effectively.
The prior art has already recognized one or more of these problems. A lot of attempts have been made to concentrate the cobalt there where it belongs, namely in or on the coating of the steel wires or steel cords.
Yet in 1936 there was an attempt to completely replace the brass coating by a pure cobalt coating on articles for reinforcing rubber (U.S. Pat. No. 2,240,805).
U.S. Pat. No. 4,255,496 (Bekaert) discloses the use of a ternary alloy copper-cobalt-zinc coating instead of a binary alloy copper-zinc (=brass) coating. With this ternary alloy, the rate of bond degradation due to aging in hot humid conditions can be significantly reduced.
U.S. Pat. No. 4,265,678 (Tokyo Rope) teaches the use of a ternary alloy copper-zinc-cobalt coating with excellent drawability and adhesion properties.
GB-A-2 076 320 (Sodétal) teaches a thin layer of cobalt on top of a brass coating followed by a wire drawing so that there is a high gradient of cobalt on top of the brass coating.
EP-A1-0 175 632 (Goodyear) teaches a quaternary alloy coating copper-zinc-nickel-cobalt on steel elements.
Finally, WO-A1-2011/076746 discloses a steel cord with a ternary or quaternary alloy coating and with a zinc gradient. Although giving improvement with respect to adhesion, this zinc gradient involves a post-treatment of the wire or cord, meaning an extra operation step in the process.